Recently, an automatic focusing (AF) camera has been widely used. An AF camera conventionally includes a lens shutter, a single-lens reflex camera, and an automatic focusing mechanism for measuring the distance to an object to be photographed and for moving the photographing lens to an in-focus position.
In the case of a single-lens reflex AF camera, the photographing lens is included in the distance measuring optical system and a distance measurement result can be obtained as a deviation between the actual position of the photographing lens and an in-focus position. The photographing lens of the single-lens reflex AF camera need only be moved by a distance corresponding to the deviation determined as the distance measurement result irrespective of the position of the photographing lens.
In the case of compact AF camera, the distance measuring optical system is normally independent from the photographing lens and the distance measurement result is obtaiend as a distance to an object to be photographed irrespective of the actual position of the photographing lens. In a compact AF camera, it is difficult to detect the current position of the photographing lens. Consequently, the photographing lens is actually returned to a predetermined position (initial position) before the distance to the object is measured and the lens is moved from the predetermined position to the focus position.
On the other hand, the distance measuring optical system and the photographing lens can be mechanically associated with each other in a compact camera to obtain a coincidence signal. In this case, since the entire distance range from the closest focusing distance to infinity must be examined, the photographing lens must be at the predetermined position before the distance is measured. That is, in either case, the photographing lens of the compact AF camera must first be returned to a predetermined or initial position. This initial position return operation requires that a mechanism not necessary for photographing be included in its camera. In addition to added complexity and expense, the initial position return mechanism may malfunction and cause a reliability problem.
A compact AF camera normally includes a shutter release switch comprising a double switch, and a photographing sequence is performed in the following manner. The initial depression of the shutter release switch, a first switch is turned on, distance measurement and photographing lens control operations are repeatedly performed so as to continuously follow an object to be photographed. Further depression of the shutter release switch turns on a second switch to activate the shutter. If the initial position return operation is performed in this photographing sequence, the photographing lens control time is prolonged which may actually disable contiuous following of the object and result in a very cumbersome photographing lens movement for the user.
This also applies to the case of continuous photographing wherein the distance to an object to be photographed continuously varies. If the photographing lens is moved by an amount corresponding to changes in the distance, no problem will result. However, if the initial position return operation is performed during the photographing lens movement, the photographing lens position becomes discontinuous and prolongs focusing time.
As described above, the initial position return operation of the photographing lens is not necessary for photographing and is not suitable for following changes in an object to be photographed. For this reason, Japanese Patent Publication No. 54-9054 describes a method which does not require the initial position return operation. Another method utilizes a system generally called a position control servo system.
According to the method described in Japanese Patent Publication No. 54-9054, automatic focusing is performed by driving the photographing lens by a motor associated with a distance measuring element. In this method, since the measuring element is a divided photoelectric converting element, the direction of movement to a focused position and the focus condition can both be detected, thus, eliminating useless movement. Since the out-of-focus amount, i.e., a deviation is unknown, focusing cannot be performed only with the measurement results, so that repeated or conntinuous distance measurement is required.
In addition, in a method wherein a photographing lens driving motor is stopped when focusing is completed by the continuous distance measurement, a photographing lens must be returned along the opposite direction because the motor overruns the focused position. This operation is repeated to control the photographing lens. Therefore, a "hunting" phenomenon sometimes occurs. In order to prevent this phenomenon, the photographing lens must be moved at low speed or a mechanism must be provided to hold the photographing lens when it reaches a target position, so that the method is not suitable for a compact camera that requires rapid photographing lens movement during continuous photographing.
Another method that does not require the initial position return operation of the photographing lens combines the active distance measuring method and the photographing lens position control servo system. The position control servo system of this method can be simply constituted by using a photographing lens driving motor and a potentiometer for detecting a position of the photographing lens. However, it is difficult to adopt this method to a compact camera because a potentiometer as a position detector is not small in size and is expensive.
Furthermore, an absolute detection code pattern may be used as a position detector. However, a high-resolution and absolute-valueu code type code pattern is complex and the pattern itself is large, and hence is not suitable for a compact camera.